Andromeda Is Twice as Heavy as the Milky Way

Turns out our sister galaxy is a total dark matter hog. But she wears the weight really well.

The Andromeda galaxy has long been considered the Milky Way's "sister," due to the the pair's comparable age, shape, and shared future as a mega-galaxy. And though Andromeda boasts about one trillion stars—over twice the Milky Way's stellar population—there has been a longstanding assumption that the galaxies are around the same weight.

But a team of astronomers based out of the University of British Columbia have upended that consensus in a new paper. The team concluded that Andromeda is almost two times as heavy as our own galaxy, mostly because it has stockpiled double the amount of dark matter of the Milky Way. The full paper is available in the July edition of the Monthly Notices of the Royal Astronomical Society.

It's unclear how Andromeda got its spiraled hands on so much dark matter, but co-author Yin-Zhe Ma has a few speculative guesses. "In the early universe, the universe was almost smooth, but there were random fluctuations in one place or another," he told to me in a phone interview.

"These regions might attract more materials, giving Andromeda a slightly stronger gravity center. It could be a random process," he said. "It could also be [that Andromeda] formed slightly earlier than the Milky Way, so it might have had more time to accumulate material."

Ma and his colleagues were able to model the relative weight of the galactic sisters by examining the smaller satellite galaxies surrounding them. "We built computer models to simulate the two galaxies as a dumbbell structure in an expanding universe," he said.

"The model showed the movement of small satellite galaxies around the larger galaxies. We measured the speed, position, and motion of these satellite galaxies to infer the structure and mass of the Milky Way and Andromeda." And voilà—these simulations revealed that Andromeda is packing dark matter like a champ.

The discovery that Andromeda is by far the heavier sister is intriguing on its own merits, but the study has broader cosmological implications as well. According to Ma, it marks the first time cosmic expansion has been observed on such a local scale. Of course, "local" still means "intergalactic," but that scale is relatively provincial when it comes to modeling the expanding universe.

"It turns out that the closer a satellite galaxy is, the more impact it has on our own galaxy," Ma said. "The further the galaxy, the greater the impact of cosmic expansion. So we observed an interplay between the local gravity and the global cosmic expansion," in the simulations of the Local Group.

As with any research into the elusive nature of dark matter, the UBC team's study inspires a host of new questions. Ma looks forward to tackling them with many of the same methods his team employed in this paper.

"We have been developing these simulations to see how dark matter accumulated and accreted onto the center the two galaxies, and therefore we can infer the structure of dark matter around these galaxies," he told me.

"The next step is to investigate how these structures are related to the motion and position of the satellite galaxies, which are the test particles we are using in these studies. This will lead to a better understanding of the properties of dark matter."